This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A179278 #26 Aug 22 2025 08:43:00
%S A179278 1,1,1,4,4,6,6,8,9,10,10,12,12,14,15,16,16,18,18,20,21,22,22,24,25,26,
%T A179278 27,28,28,30,30,32,33,34,35,36,36,38,39,40,40,42,42,44,45,46,46,48,49,
%U A179278 50,51,52,52,54,55,56,57,58,58,60,60,62,63,64,65,66,66,68,69,70,70,72
%N A179278 Largest nonprime integer <= n.
%F A179278 For n > 3: a(n) = A113523(n) = A014684(n);
%F A179278 For n > 0: a(n) = A113638(n). - _Georg Fischer_, Oct 12 2018
%F A179278 A005171(a(n)) = 1; A010051(a(n)) = 0.
%F A179278 a(n) = A018252(A062298(n)). - _Ridouane Oudra_, Aug 22 2025
%e A179278 From _Gus Wiseman_, Dec 04 2024: (Start)
%e A179278 The nonprime integers <= n:
%e A179278 1 1 1 4 4 6 6 8 9 10 10 12 12 14 15 16
%e A179278 1 1 4 4 6 8 9 9 10 10 12 14 15
%e A179278 1 1 4 6 8 8 9 9 10 12 14
%e A179278 1 4 6 6 8 8 9 10 12
%e A179278 1 4 4 6 6 8 9 10
%e A179278 1 1 4 4 6 8 9
%e A179278 1 1 4 6 8
%e A179278 1 4 6
%e A179278 1 4
%e A179278 1
%e A179278 (End)
%t A179278 Array[# - Boole[PrimeQ@ #] - Boole[# == 3] &, 72] (* _Michael De Vlieger_, Oct 13 2018 *)
%t A179278 Table[Max@@Select[Range[n],!PrimeQ[#]&],{n,30}] (* _Gus Wiseman_, Dec 04 2024 *)
%o A179278 (PARI) a(n) = if (isprime(n), if (n==3, 1, n-1), n); \\ _Michel Marcus_, Oct 13 2018
%Y A179278 Cf. A014684, A113523, A113638, A062298.
%Y A179278 For prime we have A007917.
%Y A179278 For nonprime we have A179278 (this).
%Y A179278 For squarefree we have A070321.
%Y A179278 For nonsquarefree we have A378033.
%Y A179278 For prime power we have A031218.
%Y A179278 For non prime power we have A378367.
%Y A179278 For perfect power we have A081676.
%Y A179278 For non perfect power we have A378363.
%Y A179278 A000040 lists the primes, differences A001223.
%Y A179278 A002808 lists the composite numbers, differences A073783.
%Y A179278 A018252 lists the nonprimes, differences A065310.
%Y A179278 A095195 has row n equal to the k-th differences of the prime numbers.
%Y A179278 A113646 gives least nonprime >= n.
%Y A179278 A151800 gives the least prime > n, weak version A007918.
%Y A179278 A377033 has row n equal to the k-th differences of the composite numbers.
%K A179278 nonn,easy,changed
%O A179278 1,4
%A A179278 _Reinhard Zumkeller_, Jul 08 2010
%E A179278 Inequality in the name reversed by _Gus Wiseman_, Dec 05 2024